1. Field of the Invention
The present invention relates to an apparatus for sensing the position of a switched reluctance motor (SRM), and more particularly to an apparatus for sensing the position of a switched reluctance motor (SRM) which detects a relative position of a rotor through concentric slit groups formed along different concentric circumferences of a sensor plate and compensates the position of the rotor through a compensating slit which is formed on the sensor plate.
2. Description of the Related Art
Generally, a switched reluctance motor (SRM) is a kind of a reluctance motor which mainly includes: a polyphase stator which generates a magnetic force by binding an armature coil; a rotor which is rotated by a magnetic attractive force generated according to relative positions of a tooth and the magnetic force generated by the stator; and a position detecting unit which has a position detection sensing unit and a sensor plate and detects the position of the rotor by sensing a position detection pulse by a predetermined angle resolution as the position of the rotor varies. Here, a plurality of teeth are symmetrically formed at the rotor, and the armature coil symmetrically binds each of the polyphase stator. The position detection sensing unit outputs the position detection pulse by detecting the position of the rotor and synchronizes with the position detection pulse, thereby successively driving the polyphase armature coils.
The power supplied to the armature coil which is bound to the polyphase stator is controlled by a switching element. At this time, by successively varying the excitation state between the rotor and the stator as the input pulse signal is applied to a control terminal of the switching element by synchronizing with the position detection pulse of the position detecting unit, a forward rotating torque corresponding to the input pulse signal can be generated at the rotor by the magnetic attractive force. Moreover, in the case that a specific excitation state is not varied, it is possible to stop the rotor at a predetermined position. In addition, by controlling the phase of the inputted pulse signal which is applied to the switching element based on the position where the inductance reaches at its maximum, an inverse rotating force can be generated. As described above, it is possible to widely apply the control operation in the various directions to the washing machine, etc.
At this time, to successively vary the excitation state of each phase, it is essential to sense the position of the rotor.
There are many prior arts which have been disclosed relating to the SRM, such as U.S. Pat. No. 4,748,387, "DC brushless motor driving method and apparatus for accurately controlling starting position of rotor" patented on May 1988; U.S. Pat. No. 4,990,843, "Reluctance motor" patented on February 1991; U.S. Pat. No. 5,111,095,"Polyphase switched reluctance motor" patented on May 1992; U.S. Pat. No. 5,461, 295,"Noise reduction in a switched reluctance motor by current profile manipulation" patented on October 1995; and U.S. Pat. No. 5,539,293, "Rotor position encoder having features in decodable angular position" patented on July 1996.
FIG. 1 is a view illustrating the structure of the sensor plate for sensing the position of the rotor in the conventional SRM. As shown in the drawing, at the sensor plate 10, there are a first concentric slit group 12 having a plurality of slits which are perforated to have a predetermined angle resolution along the concentric circumference having a first radius, centering around the rotary shaft 11 of the rotor; and a second concentric slit group 13 having a plurality of slits which are perforated to have a predetermined angle resolution along the concentric circumference having a second radius smaller than the first radius, centering around the rotary shaft 11 of the rotor. Generally, slits forming the first concentric slit group 12 and the second concentric slit group 13 are misaligned each other. As the sensor plate 10 is located at the upper end of the rotary shaft 11 of the rotor, it is rotated together with the rotor when the rotor of the motor is rotated. Here, the reason the slit groups each having a plurality of slits are used is to enhance the angle resolution for sensing the position of the rotor.
The apparatus for sensing the position of the conventional SRM includes the sensor plate 10 and the position detection sensing unit which senses the position of the rotor through the position detection pulse which is discretely generated by the light which optically penetrates the slits formed through the sensor plate 10.
To detect the position detection pulse, the SRM includes the position detection sensing unit which is an optical sensor having a light emitting element (not illustrated) and a light receiving element (not illustrated) therein. The light emitted from the light emitting element is discretely incident upon the light receiving element by penetrating the slits of the sensor plate 10 as the sensor plate 10 rotates along with the rotor. At this time, the light receiving element generates and outputs a predetermined position detection pulse.
A microcomputer (not illustrated) presumes the position of the rotor by receiving the position detection pulse which is outputted from the position detection sensing unit. After that, by controlling the phase of the inputted pulse signal which is applied to a control terminal of a switching unit of a motor driving unit, the microcomputer can rotate the rotor forward and inversely or stops the rotor at a specific position.
In the case that the position detection sensing unit of the conventional SRM includes a single position sensor, as it is difficult to fully obtain the information on the position of the rotor which is necessary for multiphase control, the SRM generally uses a plurality of position detection sensing units.
As described above, when the position detection sensing unit for detecting the position of the rotor outputs the position detection pulse by a predetermined angle resolution as the position of the rotor varies, the position where the armature coil is turned on is synchronized with the position detection pulse. At this time, the time when each armature coil is turned on influences decisively on the function of the SRM.
However, in the case of the apparatus for sensing the position of the conventional SRM, when the position detection pulse detected from the first concentric slit group 12 and the second concentric slit group 13 is distorted or becomes irregular or this distortion is accumulated, it causes bad influences on the operation of the SRM.
In other words, the distortion of the position detection pulse causes large rotating torque ripple relatively. In addition, it declines the rotating torque and causes noise. As a result, it declines the efficiency of the SRM.